1. Field of the Invention
The present invention relates to a driving circuit for a light emitting diode.
2. Description of the Related Art
Conventionally, a thin liquid crystal display device is used for a television and a navigation apparatus. As a backlight source of the liquid crystal display device, a light emitting diode is used because the light emitting diode has various advantages compared with a cold cathode tube. For example, the light emitting diode has a better environment property, the light emitting diode can be driven at a lower voltage, and the light emitting diode has a better low-temperature property.
The liquid crystal display device is generally configured so that a brightness of the backlight source, that is, a brightness of the liquid crystal display device can be changed based on a change in a surrounding brightness between day and night or a user setting. For example, JP-A-2005-261009 discloses a circuit in which a boost voltage of a boost circuit is applied to a light emitting diode, electricity is supplied to the light emitting diode, and when a boost operation is stopped and an applied voltage is reduced, the supply of electricity to the light emitting diode is stopped. The boost operation of the boost circuit is activated and deactivated based on a duty ratio of a brightness control signal, and thereby the brightness of the light emitting diode is controlled.
When a state is changed from a state where electricity is not supplied to the light emitting diode to a state where electricity is supplied to the light emitting diode, a large current (hereafter, called inrush current) instantaneously flows to the light emitting diode. Due to the inrush current, an input voltage applied to the light emitting diode is changed, and thereby a noise (hereafter, called a dimming noise) generates in the power source line. The dimming noise may affect an operation of other device coupled with the power source line and may cause an electromagnetic noise.
The above-described circuit includes a soft start circuit for restricting a generation of the inrush current. When an operation of a switching element for activating and deactivating the boost operation of the boost circuit is started, the soft start circuit gradually changes an output duty of a drive circuit which operates the switching element. Accordingly, a boost voltage output from the boost circuit is gradually increased and an inrush current can be restricted.
The above-described drive circuit outputs the output duty based on a magnitude relation between a voltage level and a triangular wave. The voltage level changes based on a comparison result of a feedback voltage, which corresponds to electric current flowing to the light emitting diode, and a reference voltage. The soft start circuit gradually increases the voltage level changed based on the comparison result of the feedback voltage and the reference voltage to a High level. Accordingly, the output duty depending on the magnitude relation between the voltage level and the triangular wave gradually changes.
When the output duty is gradually changed by the soft start circuit, a circuit configuration and a circuit design may be complicated, and a production cost may be increased.